Major advances have been made in the last 25 years in our understanding of the pathogenesis of childhood asthma. Strong evidence from several birth cohorts indicates that most cases of persistent childhood asthma start in the first 3 years o life; that the disease first manifests as rhinovirus-positive wheezing lower respiratory illnesses (WLRI); and that it is strongly associated with early allergic sensitization. It has also been show that both the non-reversible changes in lung function associated with the more severe forms of childhood asthma and the initial signs of airway remodeling first appear during the preschool years. Recent epidemiological studies have shown that settings and dwellings that increase bacterial exposure in early life are protective against early WLRI and asthma, but the specific components responsible for this protection are unknown. Oral lyophilized, detoxified extracts of eight pathogenic bacteria (Bronchovaxom(r)) have been used empirically in Europe for the prevention of respiratory illnesses. Recent studies in preschoolers with asthma-like symptoms have suggested that Bronchovaxom has a strong effect in the prevention of WLRI. We propose the Oral Bacterial Extracts (ORBEX): Primary Prevention of Asthma and Wheezing in Children trial to test the hypothesis that Bronchovaxom, given to 6-18 month old children at high risk for asthma, can prevent the development of persistent wheezing by age 3.5-4.5 years. We will perform a randomized, double-blind, placebo-controlled, two arm clinical trial with a total of 1076 children with the following specific aims: 1. To evaluate if Bronchovaxom(r) given to high risk infants for 10 days monthly for two consecutive years can increase time to occurrence of the first WLRI episode during a third observation year after therapy (primary outcome) and whether the rate of WLRI will be reduced after therapy. 2. To determine if: Bronchovaxom(r) can reduce the time to occurrence and the rate of WRLI while the participants are receiving study drug during the initial two years of the study; can reduce the time to occurrence and rates of severe WLRI both during therapy and during the observation year after therapy; can reduce asthma-like symptoms using `episode-free days' during both the two years on study drug and during the observation year after therapy. 3. To evaluate the safety and tolerability of the sustained use of Bronchovaxom(r) in infancy through the preschool years.